Although prior treatment with subimmunogenic amounts of some preparations of bacterial lipopolysaccharide (LPS) results in the development of immunological memory, prior exposure to others results in profound unresponsiveness or suppression. The results of preliminary experiments suggest that the later may be mediated by a subset of T cells possessing the Lyt2 cell-surface antigen. Extracts of mannan derived from Candida albicans can both enhance and suppress antibody responses to unrelated antigen. These immunomodulatory effects can be separated on the basis of molecular size and charge, indicating that they are mediated by distinct components present in extracts of mannan.